The present invention relates to an insulating composition for an electric power cable which comprises a crosslinkable ethylene polymer. The present invention also relates to an electric power cable comprising a conductor surrounded by an inner semiconducting layer, an insulating layer, and an outer semiconducting layer
Electric power cables for medium voltages (6-69 kV) and high voltages ( greater than 69 kV) normally include one or more metal conductors surrounded by an insulating material like a polymer material, such as an ethylene polymer. In power cables the electric conductor is usually coated first with an inner semiconducting layer followed by an insulating layer, then an outer semiconducting layer followed by water barrier layers, if any, and on the outside a sheath layer. The layers of the cable are based on different types of ethylene polymers which usually are crosslinked.
A power cable of the above type is normally produced in the following way:
Three layers, one inner semiconductive layer, one insulating layer and one outer semiconducting layer, are extruded onto a conductor using a triple head extruder. In this construction the insulation layer is imbedded inbetween the semiconductive layers like a sandwich construction. The insulation layer itself is normally one single layer. The thickness of the different layers depend on the gradient and the rating that the cable is exposed to. Typical values for the thickness of a MV/HV (medium and high voltage) construction are the following: the semiconductive layers are about 0.5-2 mm each and the insulation layer about 3-30 mm.
The three layers are normally extruded onto the conductor at a low temperate below 135xc2x0 C.) in order to prevent the crosslinking reactions from taking place during the extrusion process. After the extrusion step the construction is crosslinked in a pressurized vulcanizing tube at an elevated temperature.
LDPE (low density polyethylene), i.e. polyethylene prepared by radical polymerization at a high pressure and crosslinked by adding a peroxide in connection with the extrusion of the cable, is today the predominant cable insulating material. Radical polymerization results in long chain branched polymers having a relatively broad molecular weight distribution (MWD). This in turn results in desirable rheological properties with regard to their application as insulating materials for electric power cables.
A limitation with LDPE lies in the fact that it is made by radical polymerization. Radical polymerization of ethylene is carried out at high temperatures of up to about 300xc2x0 C. and at high pressures of about 100-300 MPa. To generate the high pressures needed energy consuming compressors are required. Considerable investment costs are also required for the polymerization apparatus which must be able to resist the high pressures and temperatures of radical initiated high pressure polymerization.
With regard to insulating compositions for electric power cables it would be desirable both from a technical and an economical point of view if it were possible to make an ethylene polymer with the advantageous properties of LDPE, but which was not made by radical polymerization. This would mean that insulation for electric cables could be made not only at plants for high pressure polymerization of ethylene, but also at the many existing plants for low pressure polymerization of ethylene. In order to be a satisfactory replacement for LDPE such a low pressure material would have to fulfill a number of requirements for insulating materials, such as good processability, high dielectric strength and good crosslinking properties. It has turned out, though, that for various reasons existing low pressure materials are not suitable as replacement for LDPE as insulating material for electric cables.
Thus, conventional high density polyethylene (HDPE) produced by polymerization with a coordination catalyst of Zieger-Natta type at low pressure has a melting point of about 130-135xc2x0 C. When a HDPE is processed in an extruder the temperature should lie above the melting point of 130-135xc2x0 C. to achieve good processing This temperature lies above the decomposition temperature of the peroxidos used for the crosslinking of insulating ethylene polymer compositions. Dieumyl peroxide e.g. which is the most frequently used crosslinking peroxide starts to decompose at a temperature of about 135xc2x0 C. Therefore, when HDPE is processed above its melting temperature in an extruder the crosslinking peroxide decomposes and prematurely crosslinks the polymer composition, a phenomenon referred to as xe2x80x9cscorchingxe2x80x9d. If, on the other hand the temperature is kept below the decomposition temperature of the peroxide then the HDPE will not melt adequately and unsatisfactorily processing will result.
Further, ethylene copolymers made polymerization with a coordination catalyst at low pressure, like linear low density polyethylene (LLDPE) are unsuitable due to poor processability. The processability may be improved by polymerizing the LLDPE in two or more steps (bimodal or multimodal LLDPE), but such LLDPE includes high melting HDPE fractions or components, particularly when the polymerization is carried out with conventional Ziegler-Natta catalysts, which makes LLDPE unsuitable for the same reason as conventional HDPE.
In this connection WO 93/04486 discloses an electrically conductive device having an electrically conductive member comprising at least one electrically insulating member. The insulating member comprises an ethylene copolymer with a density of 0.86-0.96 g/cm3, a melt index of 0.2-100 dg/min, a molecular weight distribution of 1.5-30, and a composition distribution breadth index (CDBI) greater than 45%. The copolymer of this reference is unimodal as opposed to multimodal.
WO 97/50093 discloses a tree resistant cable comprising an insulation layer further comprising a multimodal copolymer of ethylene, said copolymer having a broad comonomer distribution as measured by TREF, a low WTGR value and specified MFR and density values. More over, a low dissipation factor is disclosed. The document does not discuss the problem of premature decomposition of the crosslinking peroxide.
EP-A-743161 discloses a process for coextruding an insulation layer and a jacketing layer on a conductive medium. The insulation layer is a metallocene based polyethylene having a narrow molecular weight distribution and a narrow comonomer distribution. The document further reveals that the extrusion of the narrow molecular weight polymer at a low temperature is likely to lead to melt flow irregularities (so called melt fracture). This problem can be overcome by coextruding the insulation and the jacketing layer simultaneously on the conductor.
WO 98/41995 discloses a cable where the conductor is surrounded by an insulation layer comprising a mixture of a metallocene based PE having a narrow molecular weight distribution and a narrow comonomer distribution and a low density PE produced in a high pressure process. The addition of LDPE in metallocene PE is necessary to avoid the melt flow irregularities, which are the result of the narrow molecular weight distribution of the metallocene PE.
In view of the above it would be an advantage if it was possible to replace crosslinkable LDPE made by radical initiated polymerization as a material for the insulating layer of electric power cables by an ethylene polymer made by coordination catalyzed low pressure polymerization. Such a replacement polymer should have rheological properties, including processability similar to those of LDPE. Further, it should have a low enough melting temperature to be completely melted at 125xc2x0 C. in order to avoid xe2x80x9cscorchxe2x80x9d due to premature decomposition of the crosslinking peroxide.
It has now been discovered that LDPE may be replaced as a crosslinkable material for the insulation layer of electric cables by a crosslinkable ethylene copolymer made by coordination catalyzed low pressure polymerization which ethylene copolymer is a multimodal ethylene copolymer with specified density and viscosity and with melting temperature of at most 125xc2x0 C.
More particularly the present invention provides an insulating composition for an electric power cable which comprises a crosslinkable ethylene polymer, characterized in that the ethylene polymer is a multimodal ethylene copolymer obtained by coordination catalyzed polymerization of ethylene copolymer and at least one other alpha-olefin in at least one stage, said multimodal ethylene copolymer having a density of 0.890-0.940 g/cm3, a MFR2 of 0.1-10 g/10 min a MWD of 3.5-8, a melting temperature of at most 125xc2x0 C. and a comonomer distribution as measured by TREF, such that the fraction of copolymer eluted at a temperature higher than 90xc2x0 C. does not exceed 10% by weight, a said multimodal ethylene copolymer including an ethylene copolymer fraction selected from (a) a low molecular weight ethylene copolymer having a density of 0.900-0.950 g/cm3 and a MFR2 of 25-500 g/10 min, and (b) a high molecular weight ethylene copolymer having a density of 0.870-0.940 g/cm3 and a MFR2 of 0.01-3 g/10 min.
Preferably, the polymer has a viscosity of 2500-7500 Pa.s at 135xc2x0 C. and a shear rate of 10 sxe2x88x921 1000-2200 Pa.s at 135xc2x0 C. and a shear rate of 100 sxe2x88x921 250-400 Pa.s at 135xc2x0 C. and a shear rate of 1000 sxe2x88x921.
A density in the lower part of the range, i.e. 0.890-0.910 g/cm3 is aimed at when a very flexible cable is desired. Such cables are suitable for applictions in cars, mines and the building industry. These low densities are only possible to reach by using a single site catalyst such as a metallocene type catalyst, at least for the higher molecular weight fraction. When densities in the range 0.910-0.940 g/cm3 are chosen, the resulting cables are stiffer, but have better mechanical strength values, and are therefore more suitable for non-flexible power supply cables.
The present invention also provides an electric power cable comprising a conductor surrounded by an inner semiconducting layer, an insulating layer, and an outer semiconducting layer, characterized in that the insulating layer comprises a crosslinked ethylene copolymer obtained by coordination catalyzed polymerization of ethylene and at least one other alpha-olefin in at least one stage, said multimodal ethylene copolymer having a density of 0.890-0.940 g/cm3, a MFR2 of 0.1-10 g/10 min, a MWD of 3.5-8, a melting temperature of at most 125xc2x0 C. and a comonomer distribution as measured by TREF such that the fraction of copolymer eluted at a temperature higher than 90xc2x0 C. does not exceed 10% by weight, and sad multimodal ethylene copolymer including an ethylene copolymer fraction selected from (a) a low molecular weight ethylene copolymer having a density of 0.900-0.950 g/cm3 and a MFR2 of 25-500 g/10 min, and (b) a high molecular weight ethylene copolymer having a density of 0.870-0.940 g/cm3 and a MFR2 of 0.01-3 g/10 min.
Preferably, the polymer has a viscosity of 2500-7500 Pa.s at 135xc2x0 C. and a shear rate of 10 sxe2x88x921 1000-2200 Pa.s. at 135xc2x0 C. and a shear rate of 100 sxe2x88x921 and 250-400 Pa.s at 135xc2x0 C. and a shear rate of 1000 sxe2x88x921.
These and other characteristics of the invention will appear from the appended claims and the following description.